I am working on a web application which uses RAP. In the application there is one bundle which contains the model which is backed by a database. I would like to create bundles which provide REST services which will make use of the model bundle.
I looked at the Application#addEntryPoint but that it just for UI contributions - not for services as such.
I also read FrankAppel's post http://www.codeaffine.com/2011/08/26/raprwt-osgi-integration/ and wonder if RWT and Felix might be the way to go. It looks promising but Felix is new to me.
Is it possible to add these REST bundles to the RAP application and set them up to handle /rest/* URLs? Or would it be more sensible to keep the 2 parts completely separate and to share the model bundle in a different way?
When using RAP, any active bundle may contribute to the usual "org.eclipse.equinox.http.registry.servlets" and "org.eclipse.equinox.http.registry.resources" extension points. You just need to make sure the name of your RAP application's entry point(s) and the paths of your ressources and servlets won't overlap.
So in practice, you can just develop your REST services as if there was no RAP component. The two will happily live side-by-side within the same servlet context.
Related
I'm new to building APIs, I made the first one using an MVC framework: codeigniter, with chris kacerguis rest implementation.
I'm not really sure this was the best think to do because I believe maybe the framework is not that "slim" or light just to API's purposes.
I plan to do a mobile App, an admin and a website so the three can consume the Api's services.
Is it a bad idea to have the API, the website and the admin on the same project? which are the pros and cons? or the best architectural approach?
Otherwise I will have: One Codeigniter project for the API and Another Codeigniter project for website and admin
thanks
You can create folders in "controllers" folder to organize your project and use the same project/env configuration :
controllers/Home.php
controllers/api/MyApi.php
controllers/admin/Admin.php
Edit : You will share models and libraries too.
In my project I realized 2 types of controller - REST and API. Admin js gui work with REST, other world work with API. You can do it simply with silex framework, a little brother of symfony.
The purpose of building a REST API so that you only have to build one project for your business model. This allows you to construct any number of applications on any platform, only requiring you to consume the API in different ways. This essentially separates/decouples the user interface from the business logic, and vice versa.
You should create separate projects for the REST API and each UI project should also be separated projects. This allows you to change the underlying code, language and platform in any one of the projects without breaking any of the other projects as long as the API signatures remain the same.
For example, you could have a live version of your website built using Codeignitor while developing another septate project using AngularJS. When your AngularJS project is complete you would simply swap out the project on your server (or create an entirely new website or server) still allowing you to use the other if required. Additionally, you may decide that you would like to move the API onto a different platform, language or database, develope it and swap the implementation when finished causing no changes to any of your UI projects assuming you have not changed the API signatures.
When UI is supplied by Angular web application, the responsibility of the other micro service applications gets limited to supplying JSONS. But they still need to support ReST paths and cannot be a monolithic giant. Meaning, can't just make those as JARs and bind it as dependency to some WAR app. Is there a better way to do this than keeping the WAR applications that are just going to supply JSONs as UI less WARs for the sake of making sure that they are not mixed up?
I'm in the process of exposing some of the data generated in one OSGi bundle with a REST interface so it can be consumed with typical http requests afterwards by anyone. To do this I'm using a combination of apache-cxf, JAX-RS and Jetty (for testing purposes mostly).
My whole application is distributed in two ways:
Core: Runs a set of minimal bundles as an OSGi framework.
Full: Core + plus a set of plugins to be used within Eclipse, in a graphical way, to put it that way.
While in the Core launch configuration I can make this work without any problems, have a look at the MANIFEST:
Require-Bundle: cxf-dosgi-ri-singlebundle-distribution;bundle-version="1.3.1",org.eclipse.core.runtime, org.eclipse.osgi.services;bundle-version="3.3.0", org.mortbay.jetty.server;bundle-version="6.1.23", org.mortbay.jetty.util;bundle-version="6.1.23"
When I try to launch the Full distribution, jetty server does not seem to start and thus, the RESTful service is not working properly. Full distribution comes with a lot more plugins, mostly related with wizards, perspectives and so on. Root of this problem is somehow related with the cxf-dosgi-ri-singlebundle-distribution bundle, and can be tackled by "forcing" auto-start of this bundle to true, instead of default (check image below):
While this is OK, my question is ...why is this happening? And second and most important, I distribute the Full distribution via an update site, so users can download it, open perspective and start using it, that said, will this affect the final distribution? Is there a way I can control this auto-start=true for the apache-cxf bundle in the update site?
Hope I can make myself clear...
Thanks!
I don't like answering my own questions, but since I'm getting no feedback and maybe someone else has the same problem...here it goes
I decided to change the whole design of my restful bundle, by not using apache-cxf, since it is really not needed. All I need to do what I want is the embedded Jetty server and javax for dealing with HttpRequests and HttpResponses. Thus, I get rid of dependecies with third party libraries, leveraging the launching problems at the same time.
Got the ideas from these official sites in Eclipe:
Embedding an HTTP server in Equinox
Writing a bundle-based server application
Besides, a more hands-on tutorials can be also found here:
OSGi as a Web Server application
Extending Eclipse: displaying HTML content from a bundled archive
Special thanks to both authors for the enlightenment :).
I am trying to get my head around OSGi Services. The main question I keep asking myself is: What's the benefit of using services instead of working with bundles and their exported packages?
As far as I know it seems the concept of Late Binding has something to do with it. Bundle dependencies are wired together at bundle start, so they are pretty fixed I guess. But with services it seems to be almost the same. A bundle starts and registers services or binds to services. Of course services can come and go whenever they want and you have to keep track of these chances. But the core idea doesn't seem that different to me.
Another aspect to this seems to be that services are more flexible. There could be many implementations for one specific Interface. On the other hand there can be a lot of different implementations for a specific exported package too.
In another text I read that the disadvantage of using exported packages is that they make the application more fragile than services. The author wrote that if you remove one bundle from the dependency graph other dependencies would no longer be met, thus possibly causing a domino effect on the whole graph. But couldn't the same happen if a service would go offline? To me it looks like service dependencies are no better than bundle dependencies.
So far I could not find a blog post, book or presentation that could clearly describe why services are better than just exposing functionality by exporting and importing packages.
To sum my questions up:
What are the key benefits of using OSGi Services that make them superior to exporting and importing packages?
Addition
I have tried to gather further information about this issue and come up with some kind of comparison between plain export/import of packages and services. Maybe this will help us to find a satisfying answer.
Start/Stop/Update
Both, bundles (hence packages) and services, can be started and stopped. In addition to that they can be kind of updated. Services are also tied to the bundle life cycle itself. But in this case I just mean if you can start and stop services or bundles (so that the exported packages "disappear").
Tracking of changes
ServiceTracker and BundleTracker make it possible to track and react to changes in the availability of bundles and services.
Specific dependencies to other bundles or services.
If you want to use an exported package you have to import it.
Import-Package: net.jens.helloworld
Would net.jens.helloworld provide a service I would also need to import the package in order to get the interface.
So in both cases their would be some sort of "tight coupling" to a more or less specific package.
Ability to have more than one implementation
Specific packages can be exported by more than one bundle. There could be a package net.jens.twitterclient which is exported by bundle A and bundle B. The same applies to services. The interface net.jens.twitterclient.TwitterService could be published by bundle A and B.
To sum this up here a short comparison (Exported packages/services):
YES/YES
YES/YES
YES/YES
YES/YES
So there is no difference.
Additionally it seems that services add more complexity and introduce another layer of dependencies (see image below).
alt text http://img688.imageshack.us/img688/4421/bundleservicecomparison.png
So if there is no real difference between exported packages and services what is the benefit of using services?
My explanation:
The use of services seems more complex. But services themselves seem to be more lightweight. It should be a difference (in terms of performance and resources) if you start/stop a whole bundle or if you just start and stop a specific service.
From a architectural standpoint I also guess that bundles could be viewed as foundation of the application. A foundation shouldn't change often in terms of starting and stopping bundles. The functionality is provided by services of this packages in some kind of dynamic layer above the "bundle layer". This "service layer" could be subject to frequent changes. For example the service for querying a database is unregistered if the database is going offline.
What's your opinion? Am I starting to get the whole point of services or am I still thinking the wrong way? Are there things I am missing that would make services far more attractive over exported packages?
Its quite simple:
Bundles are just providing classes you can use. Using Imports/Exports you can shield visibility and avoid (for example) versioning conflicts.
Services are instances of classes that satisfy a certain contract (interfaces).
So, when using Services you don't have to care about the origin of a implementation nor of implementation details. They may even change while you are using a certain service.
When you just want to rely on the Bundle Layer of OSGi, you easily introduce crosscutting dependencies to concrete implementations which you usually never want. (read below about DI)
This is not an OSGi thing only - just good practice.
In non OSGi worlds you may use Dependency Injection (DI) frameworks like Guice, Spring or similar. OSGi has the Service Layer built into the framework and lets higher level frameworks (Spring, Guice) use this layer. - so in the end you usually dont use the OSGi Service API directly but DI adapters from user friendly frameworks (Spring-->Spring DM,Guice-->Peaberry etc).
HTH,
Toni
I'd recommend purchasing this book. It does an excellent job explaining services and walking through the construction of a non-trivial application that makes use of OSGi Services.
http://equinoxosgi.org/
My company routinely builds 100+ bundle applications using services. The primary benefits we gain from using services are:
1) Loose coupling of producer/consumer implementation
2) Hot swappable service providers
3) Cleaner application architecture
When you start with OSGi, it is always easier to start with an export-package approach it certainly feels more java-like. But when your application starts growing and you need a bit of dynamicity, services are the way to go.
Export-package only does the resolution on startup, whereas services is an on-going resolution (which you may want or not). From a support point of view having services can be very scary (Is it deterministic? How can I replicate problems?), but it is also very powerful.
Peter Kriens explains why he thinks that Services are a paradigm shift in the same way OO was in its time. see µServices and Duct Tape.
In all my OSGi experience I haven't had yet the occasion to implement complex services (i.e. more than one layer), and certainly annotations seem the way to go. You can also use Spring dynamic module to ease the pain of dealing with service trackers. (and many other options like iPOJO, and Blueprint)
Lets consider the two following scenarios:
Bundle A offers a service which is an arithmetic addition add(x,y) return x+y. To achieve this, it exports "mathOpe package" with "IAddition interface", and registers a service within the service registry. Bundles B, C, D, ... consume this service.
Bundle A exports "mathOpe package", where we found a class Addition exposing an operation (x+y)<--add(x,y). Bundles B, C, D, ... import the package mathOpe.
Comparison of scenario 1 vs. scenario 2:
Just one implementation instance vs. many instances (Feel free to make it static!)
Dynamic service management start, stop, update vs. no management, the consumer is owning the implementation (the "service")
Flexible (we can imagine a remote service over a network) vs. not flexible
... among others.
PS: I am not an OSGI expert nor a Java one, this answer shows only my understanding of the phenomena :)
I think this excellent article could answer a lot of your questions:OSGi, and How It Got That Way.
The main advantage of using a service instead of the implementation class is that the bundle offering the service will do the initialization of the class itself.
The bundle that uses the service does not need to know anything about how the service is initialized.
If you do not use a service you will always have to call kind of a factory to create the service instance. This factory will leak details of the service that should remain private.
In a MVC application, what are some of the components that make up the application. What tools and functionality is missing that you would like to have. Regardless of the server-side language, what would you want?
I see a lot in my code where I code some much functionality that it seems should already be there. I looked at Google web toolkit and they seem to get it right. Widgets are widgets and you simply add them to your application.
For example. I work with J2EE apps but in other languages, the components are the same.
Controller Objects
Controller handlers, defined by methods in the controller objects.
Configuration files defining the URL mapping and settings.
Template server page files (e.g. JSP/ASP files).
Configuration files defining O/RM mapping between application objects and the database.
Configuration files defining the database connection properties.
JavaScript libraries (e.g. jQuery)
Logging configuration files
Resource message bundle files
Validation configuration files or code
Middleware components and objects (EJB configurations, JMS/Messaging configurations, etc).
Credit Card or other middleware connectivity APIs and libraries.
Anything else you can think of?
Built-in Unit Testing Component
I think one thing you're missing from that very exhaustive list is the automatic binding of request properties to form objects, and the saving of these objects to the session where appropriate. Form objects here being the object on the server that represents the current state of the HTML-based for displayed to the user.
I think scaffolding and automatic admin interfaces are very nice features too, that I dont want to miss ;)
You've made the assumption that all MVC applications are websites. MVC is widely used for more than just web apps so things like URL mappers, template server pages and "Server side" languages are not associated with the MVC pattern, so much as a particular implementation and adaptation of the MVC for use in web apps.